GNU: Remove duplicate and unnecessary optimization flags for gfortran

Since commit a66004bee0 (Honor CMAKE_<LANG>_FLAGS[_<CONFIG>]_INIT set in
toolchain files, 2016-07-05, v3.7.0-rc1~392^2) we've accidentally been
adding extra optimization flags instead of replacing unwanted flags.

Fixes: #25434

Modules/Compiler/GNU-Fortran.cmake

@@ -17,10 +17,6 @@ endif()
 
 set(CMAKE_Fortran_POSTPROCESS_FLAG "-fpreprocessed")
 
-# No -DNDEBUG for Fortran.
-string(APPEND CMAKE_Fortran_FLAGS_MINSIZEREL_INIT " -Os")
-string(APPEND CMAKE_Fortran_FLAGS_RELEASE_INIT " -O3")
-
 # No -isystem for Fortran because it will not find .mod files.
 unset(CMAKE_INCLUDE_SYSTEM_FLAG_Fortran)
 

Modules/Compiler/GNU.cmake

@@ -109,9 +109,14 @@ macro(__compiler_gnu lang)
   # Initial configuration flags.
   string(APPEND CMAKE_${lang}_FLAGS_INIT " ")
   string(APPEND CMAKE_${lang}_FLAGS_DEBUG_INIT " -g")
-  string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -Os -DNDEBUG")
-  string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -O3 -DNDEBUG")
-  string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -O2 -g -DNDEBUG")
+  string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -Os")
+  string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -O3")
+  string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -O2 -g")
+  if(NOT "x${lang}" STREQUAL "xFortran")
+    string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -DNDEBUG")
+    string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -DNDEBUG")
+    string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -DNDEBUG")
+  endif()
   set(CMAKE_${lang}_CREATE_PREPROCESSED_SOURCE "<CMAKE_${lang}_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -E <SOURCE> > <PREPROCESSED_SOURCE>")
   set(CMAKE_${lang}_CREATE_ASSEMBLY_SOURCE "<CMAKE_${lang}_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -S <SOURCE> -o <ASSEMBLY_SOURCE>")
   if(NOT APPLE OR NOT CMAKE_${lang}_COMPILER_VERSION VERSION_LESS 4) # work around #4462